To mention someone in a post or comment, enter "@" and the first letter of an existing account. There will appear a list of people starting with these letters.
Select the right username you want to mention and it will turn blue. Once you have uploaded the post, the user will be mentioned underneath the content. A notification will be sent if you mention someone who does not follow you. It will then be necessary for the post to be public and you have not been blocked by the user before.
To look at what you posted, go to your dashboard and tap on "My Posts".